The invention relate to a method and a device for trimming at least one side edge of a bound printed product such as book blocks, books, brochures, magazines, or similar flat objects. The printed product is positioned on a trimming table and supplied in a position ready for trimming. The printed product is held in place during the trimming, realized with a driven trimming knife, by a press die that can be raised and lowered.
Devices of the aforementioned type are known as three-way trimmers or trimmers. The devices are used for trimming along three side edges of bound printed products. To achieve the most precise cut possible, the printed products are first aligned along the spine and the side edges. The printed products are then clamped in between a trimming table and the lowered press die while pressure is applied to the flat sides. The required pressing force must at least be high enough so that during the trimming operation the printed product cannot move, relative to itself or relative to the press die. The distribution of the pressing force over the surface area of the printed product is furthermore selected such that the specific pressure exerted onto the printed product is higher in the side edge areas to be trimmed than in the remaining regions. This can be achieved by selecting a greater thickness for the edge regions of the press die than for the other locations.
It has turned out, however, in practical operations the surfaces of some printed products, which are not clamped down, can be uneven. The unevenness may be the result of waviness caused by a movement direction transverse to the bound edge of the printed product for the printed sheets, which are bound into a printed product. The unevenness may also be the result of folds generated along the side edge to be trimmed, air inclusions between the pages, and for other reasons. The top surface of the printed products positioned on the trimming table may therefore assume a concave shape. If several stacked copies are trimmed simultaneously, this defect may be cumulative on the surface of the uppermost copy.
At the start of the pressing operation, the first contact between the printed product and the press die therefore occurs in the edge regions, causing the printed products to be secured in this position between press die and the trimming table. This secured position prevents the printed product region facing the press die from moving, relative to the press die surface, during the build-up of the pressing force. The cover areas located between these contact locations are thus somewhat longer than the corresponding, straight-line section of the press die. As a result, a wave respectively may form in the cover during the continued pressing, wherein this wave may move from the outside toward the inside until it meets the wave moving from the opposite side toward the inside. The cover is consequently compressed or creased and is pressed down in this state by the press die. Permanent squeezing folds are thus generated on the top pages of the cover and the content, which can lead to reduced quality or to rejected products.